SEX IS A BATTLEFIELD

Transcription

1 SEX IS A BATTLEFIELD Strategies to end up on top! Tanya Harris, Ph.D, Embryology Lab Manager, Genea Oxford Fertility SEX SELECTION Special case of Natural Selection - an individuals ability to successfully reproduce, often at risk of non-survival. Meaning of life: reproduction and survival, don t do either badly! Battle between females and males for control of reproduction outdoing one another for control, males can potentially reproduce each copulation, not so females Species survive best if they have sex, asexual reproduction can work well but not generally in the long term for each individual Sex mixes genes; if we all had the same genes then surviving is a matter of luck. Genes that give an advantage to an individual increase overtime = Evolution through Natural Selection Can happen quickly antibiotic resistance in bacteria Gene differences come about through mutation (creates change) and sex (creates new combinations of genes) 1

2 THE SURVIVING PART IS WELL WORKED OUT BUT ATTRACTING A MATE IS DIFFICULT.. Surviving has a limited number of tried and true formulas: Live in a group Move fast Blend in Have a hard shell or sharp bits Taste yucky But reproducing is more difficult and MANY strategies have evolved to tackle this problem HUMAN ATTRACTION Humans = socially monogamous - low sperm competition, all about attracting a mate Dropping sperm counts in developed countries? Related to monogamy and low sperm competition Humans = monogamous but adulterous Extra pair paternity (cuckoldry) levels estimated to be low at 1-5% yet some studies have put extra-marital affairs at 54% for women and 74% for men (married >2 years). More likely environmental effects Attractive: Markers of health and fertility Men Like: Shorter youthful women, symmetrical face, full breasts and lips, low waist-hip ratio Women Like: Taller intelligent men, facial symmetry, masculine face, broad shoulders, V shaped torso with narrow waist. 2

3 Lingerie company Bluebella conducted a survey (not particularly scientific) & separated responses by sex (women went skinnier and men went bulkier) LOOKING GORGEOUS IN OTHER SPECIES Promiscuous females tend to have more and healthier offspring Females may choose to mate with more than one male to provide extra protection (blackbirds), prevent aggression towards offspring (lions), prevent infertility increases reproductive success of some males only Species where males are more brightly coloured tend to have higher levels of extra-pair paternity Coloured leg bands have accidentally changed the success of some males during experiments Mate guarding during fertile period benefit must outweighs costs to lost opportunities to forage, copulate with others Pair and stay close Find a virgin or prepubescent female and guard her Beetles, shrimp Prolonged or frequent copulation May reduce survival but can ensure his sperm near eggs 3

4 SPERM VS EGG! Sperm numbers represent a crude index of how difficult it is to reach the egg & how promiscuous females are Why? Massive numbers of sperm will die before they get to the egg THE BATTLEFIELD = female reproductive tract Acid vagina Physical barriers (Cervix or cloaca) Immune defence Some females remove sperm or eject seminal fluid after copulation A hostile reproductive tract favours increasing sperm numbers and factors in seminal fluid to help sperm survive Human sperm is poor quality compared to other primates low reserves & lower numbers BUT human males do have a high penis size relative to body size HUMANS EXPERIENCE LOW LEVELS OF SPERM COMPETITION Humans sit somewhere between Chimpanzees and Gorillas Female Chimps are highly promiscuous & mate indiscriminately during their fertile period (2-3 days) each month, x per pregnancy Gorillas live in harems and females will generally mate only with the alpha male Level of sperm competition relates to sperm numbers and also size of the testes. Chimp testicles are 3x larger than human, male gorilla has 2 inch penis 4

5 OTHER STRATEGIES FOR ATTRACTING A MATE Offer a gift= energy intensive so males are often picky about which females they choose Nutritional- food or sperm itself - Some crickets grow an extra pair of delicious wings Stones for nest Adele penguins Ultimate gift = himself (Sexual Cannibalism) Male praying mantis plays a game of freeze leaping onto female s back. If mistimed she eats his head. Headless he can copulate with her in even more frenzied fashion Male redback spiders allows themselves to be consumed after copulation Copulation is 2x longer she is not as interested in mating after eating Many spiders evolved ways of avoiding being eaten Spurs on fangs to wedge open jaws, tie the female up or give her drugs to subdue her Male scorpions whack the female with their tails and wolf spiders throws female in air and runs off PROMISCUOUS FEMALES & CREATIVE MALES Ornate Penises Helps to rid previous rivals sperm or to stimulate females Maintain contact to prolong copulation (lock and key - pigs) and get closer to egg E.g Argentinian lake duck with 8 penis with brush hairs on the end False penis Local African community know it to be used to carry sticks back to make the nest, however, foreplay is long and seems to help with female acceptance of his courtship and ejaculation into her cloaca Many penises Some marine flatworms have dozens Snakes & lizards have two independent penises which they use alternately, each has its own testis and epididymis frequent copulation 5

6 PROMISCUOUS FEMALES & CREATIVE MALES Chastity Belts: Popular invention: bats, rats, worms, snakes, spiders, butterflies, fruit flies, guinea pigs, squirrels, chimpanzees Most are cement-like plugs (hardened seminal fluid) In turn females and even males of many of these species have evolved ways to remove the plug Be agile enough to reach around, hooks Ultimate sacrifice to reproduce Leave parts of your genitals behind to physically prevent other males mating Male bees snap off their genitals so that others cant mate with the queen extremely poor opportunities to mate. 1 female : males No-one else to mate with and they will die anyway SEX MANIA, FEMALE PHALLUSES AND MANNY S Induced Ovulators Stimulation of the female is required in order for her to ovulate, the eggs to fertilise, or for pregnancy to be ongoing Rabbits, ferrets, domestic cats and rats Females control copulation The clitoris of a Hyena is shaped and functions like a penis (initially thought to be hermaphrodites) Males grovel for opportunity to mate and must adopt difficult position to copulate so females have total control of which males are successful Downside they must give birth through it.ouch! Male takes care of the babies Male Seahorses and Pipefish have brood pouch or plate which female transfers her eggs into Guarantees his sperm fertilises the eggs DNA confirms complete sexual monogamy 6

7 INTERNAL FERTILISATION BUT NOT AS WE KNOW IT. Cooperative mating Slugs & snails hermaphrodites Avoids being exploited for sperm but not receiving any If knot one must gnaw off his/partners phallus and become female only afterwards Marine flatworms attempt to stab each other anywhere on body as no female orifice as such Traumatic insemination How ugly losers become winners Bedbugs, cephalopods (Squid, octopus, cuttlefish) Bypass female reproductive tract control but shooting sperm packet into the body of female or rival male Spermatophores absorb sea water once outside the male and explode once in the female. UNUSUAL LOOKING SPERM Unlike sperm number: the evolution of sperm shape and size is not well understood. Sperm tend to be smaller and simpler in species where eggs are fertilised outside the female s body Many species have sperm that don t follow the basic tadpole like shape Hooks, frisbees, sperm that swim in tandem, amoeboid (crawling), spermatophores (exploding packets of sperm) 7

8 GIANT SPERM General rule = more promiscuous the female the larger the sperm of the male However, size and number cant keep increasing - number usually wins out except in a few species Drosophila = 3mm long with 58 mm long sperm (blue whale sized sperm in human!) Also some beetles, ticks, shrimp, snails, frogs Unclear how giant sperm help except to fill up the female & provide nutrients? Last male preference the last sperm in is often the sperm that fertilises the greatest proportion of the eggs CONCLUSION: BATTLE BETWEEN MALES AND FEMALES FOR CONTROL OF REPRODUCTION STRATEGIES THAT EXIST ARE MANY AND VARIED FEMALES ARE NOT PASSIVE, AND MAY CHOOSE TO MATE WITH MALES OTHER THAN THEIR PARTNER 8